There are several Constantly Variable Transmissions (CVTs) that are being successfully designed and manufactured as transmissions for vehicles and other machines that require changing gear ratios.
The market is supplied generally by Hydro-mechanical CVT's and traction based CVT's.
Two major types of traction drives are used, although there are also many other proposals. One uses a belt running between adjustable pulley sheaves and the other a roller that runs between two discs with the negative shape of a toroid machined into it, generally called toroidal variators.
This invention is related to toroidal variators and the control of the rollers within them.
The toroidal drive market is dominated by two similar types of mechanism. One is generally called a Single Roller Half Toroidal Variator (SHTV) and the other a Single Roller Full Toroidal Variator (SFTV). A new design subject to International PCT Application No. PCT/AU2010/001331 called Double Roller Full Toroidal Variator (DFTV) is being developed which uses two rollers running against each other.
Both the SFTV and SHTV drives involve the use of twin discs that are machined with the negative shape of a toroid in opposing faces. Between the discs are rollers that roll against the surface of both toroidal cavities and can transfer force and power from one disc to the other. In the case of the Double Roller Full Toroidal drive the rollers are in pairs with one roller running against the other.
A special fluid called traction fluid is used for the actual force transfer. This fluid has the ability to become extremely viscous (almost solid) as it is squeezed between the rollers and the discs. Its ability to transfer force is determined by its “traction coefficient” which is similar to a static friction coefficient.
These rollers can be rotated or steered so that they contact the discs in different places and in so doing can change gear ratio in a seamless manner as they move.
In a SFTV drive the rollers centre of rotation is located on the centre of the toroidal cavity. In a SHTV drive the rollers are located off the centre of the toroid towards the centre of rotation of the toroid.
Both types use different methods of rotating the rollers to produce a ratio change. Generally they use a control pressure acting against the torque reaction force coming off the rollers. In this way the rollers are said to be torque controlled as distinct from ratio controlled.
The control pressure, if greater than the reaction torque, will move the rollers in one direction; if less, the rollers will move in the opposite direction. The movement of the rollers can be arranged to run against a cam restraint or similar arrangement and so rotate the rollers to produce a new ratio.
This type of control is described in US patent application no. 2008/0254933 A1 (Oct. 16 2008 inventor Brian Joseph Dutson), U.S. Pat. No. 5,989,150 (Nov. 23 1999 assignee Jatco Corporation) and US patent application no. 2008/0009387 A1 (Jan. 10 2008 assignee NSK Ltd.). The disclosures of which are incorporated herein by way of reference.
In some mechanisms the rollers are controlled individually (Jatco) while in others (Dutson and NSK) they are ganged together.
In most cases the toroidal discs are clamped with a clamping force that is proportional to the torque being transferred and a friction constant called the traction coefficient. Normally two toroidal cavities are provided with the input being provided by the two outside discs and the output coming off from the side using gears or chains. In this way there is no unsupported rotating force and no thrust bearing provided to support it.
Most toroidal drives use a method of roller steering, which is incorporated in the torque reaction support system to cause the rollers to move from one ratio to another. In this way very little force is actually applied to the roller to rotate it from one position to another. It must be understood that when the rollers are clamped between the discs with a large force it is not practical to simply slide them across the face of the discs in order to create a new ratio.
In a SFTV or SHTV this roller steering effect is accomplished using a roller support method in which the torque reaction force from the roller bears on a piston whose centre passes generally through a tangent of the centre of the toroidal cavity. When the piston pushes the roller forward or the roller moves back against the piston, the centre of rotation of the roller ceases to pass through the centre of rotation of the discs. It is immediately subjected to sideways forces acting in opposite directions on each disc and the roller rotates. This is usually referred to as roller steering and it is responsible for the ratio change responsiveness of this type of CVT.
FIGS. 1 and 2 shows three rollers 1 mounted between Full Toroidal Variator discs. The output disc 10 is clamped (not shown) onto the input disc 11. The input disc 11 is driven by the input shaft 12 while the output disc 10 drives shaft 13. The rollers 1 are provided with an axle 2 that runs on bearings 3. The bearings 3 are held in a yoke-like mounting 4.
The yoke 4 is connected to a piston 7 via a ball joint connection 5, 6 and shaft 9. The piston 7 runs inside an hydraulic cylinder 8. Two chambers 14, 15 can be supplied with high pressure oil to balance the torque reaction force coming off the roller 1. Although the discs always rotate in the same direction the torque reaction force can act in both directions depending on the state (accelerating or decelerating) of the mechanism being driven.
The cylinder 8 is mounted in a position where its centre axis passes generally through the centre of the toroidal cavity and is generally tangential to the circle defined by the centre of this cavity. It does, however, not lie exactly in a plane parallel to the plane of rotation of the discs but is inclined at an angle to this plane according to the castor angle 16.
It can be seen that the input and output discs 10, 11 and associated shafts 12, 13 rotate in opposite directions.
It can be seen that when in a steady state the axis of rotation of the roller 1 passes through the axis of rotation of the discs 10, 11. If the roller 1 moves forward or backward under the influence of the forces on it the axis of its rotation will no longer pass through the centre of rotation of the discs 10, 11 and it will be subject to steering forces that rotate it into a new ratio position.
This simultaneous mechanical action is brought about by the castor angle. As the roller 1 rotates under the influence of the steering forces its ball joint connection 5, 6 swings outward causing its rotational axis to coincide with the rotational centre of the discs 10, 11. When this occurs the steering action ceases and the roller 1 settles in a stable position, provided the roller reaction force and the control force are balanced.
The Half Toroid designs operate a similar arrangement but for other mechanical reasons cannot use a castor angle. Roller stability, after a steering movement, is achieved by allowing the roller 1 to move back to a central position by readjusting the control pressure that is reacting against the roller reaction. A stepper motor and cam recognizes where the roller 1 is and makes continuous adjustments to the control pressures using a feedback loop. Feedback loops are always subject to over-running targets resulting in oscillations or hunting. Complex dampening systems are often required to eliminate such oscillations that are unacceptable in most transmission applications.
In these prior art examples, all the rollers 1 are free to adopt independent ratio positions. However the positions that they do adopt are generally positions where they are all experiencing the same torque reaction force, as it is balanced against equal sized pistons 9 all subject to the same hydraulic fluid pressure. This balanced positioning is intended to ensure that all rollers 1 share an equal amount of the transmitted power. It is not theoretically possible for the rollers 1 to be out of ratio position because of this. However the inherent flexibility of the hydraulic fluids and the galleries in which it runs can cause oscillations that are difficult to dampen.
Other problems exist with this form of roller position control. Firstly, because the rollers 1 are supported on a force provided by an active hydraulic system, energy is continuously needed to provide the hydraulic pressure. The active hydraulic system must be large enough to move the rollers 1 quickly and so the size of the system becomes quite substantial requiring a power level that can often be 2%-4% of the energy passing through the CVT.
There are a large number of precision parts associated with the support system and the hydraulic controls associated with the hydraulic system.
There are mechanical constraints that limit the angle through which the rollers 1 can turn. This limits the overall ratio spread. Typically the maximum ratio spread for a SFTV is 6:1. It also limits the distance that the roller centre can be located from the disc axis of rotation, which in turn decreases its efficiency.
Another approach to the need to “steer” rollers to new ratios is described in U.S. Pat. No. 6,866,609 applied to a SHTV.
It is an aim of this invention to provide a control mechanism that controls the position of the rollers mechanically without the need for hydraulics and in a way that allows them to steer to new ratios without requiring a high level of force.
It is a further aim to create a control mechanism that accurately gangs together all of the rollers so that they adopt new ratios without any possibility of elastically induced oscillations.
It is a further aim of this invention to reduce the energy lost in the roller actuation system.
It is a further aim of the invention to improve the efficiency of the variator by locating the roller centre further from the disc rotational axis so that the degree of differential velocity at the contact points is lessened.
It is a further aim of the invention to increase the ratio spread of the variator by enabling the rollers to rotate through a greater angle creating a larger difference in the distance of the contact points from the disks rotational axis.
It is a further aim of the invention to increase the number of rollers that can be arranged within the toroidal cavity by creating a larger difference in the distance of the contact points from the disks rotational axis.